1. Field of the Invention
The invention relates to the field of pattern generation using a laser and radiant sensitive film, particularly for photolithography.
2. Prior Art
In the photolithographic fabrication of integrated circuits, films sensitive to radiant or particle energy are exposed in predetermined patterns to define circuit features. In some cases, the energy is passed through masks which contain the patterns, thereby selectively exposing a photoresist film on a semiconductor body. In other instances, the film is on a mask substrate and the film is exposed as a step in the making of the mask. Other times the direction of the radiant energy itself is controlled to define patterns in the film. This can be done as part of making a mask or to directly "write" onto the photoresist film covering a semiconductor wafer.
Several sources of radiant energy have been used, including ultraviolet light, visible light, coherent light, x-rays and electron beam (E-Beam).
In the very early days of photolithographic processing, patterns were manually cut on a large scale when compared to the final circuit, then photographically reduced to make the final masks. With today's technology, E-Beams are electrically directed to define patterns sometimes at the final scale.
There have been attempts to fabricate masks by directing laser beams and/or moving a work piece relative to laser beams. None of these attempts are commercially used. As will be seen, the present invention is directed to this area.
General pattern generation is described in U.S. Pat. Nos. 3,465,091; 4,060,816, and 4,464,030. Some aspects of UV mask making technology is described in U.S. Pat. Nos. 4,293,624 and 4,329,410. E-Beam technology is discussed in U.S. Pat. Nos. 3,679,497; 3,857,041, and 4,445,039. Laser pattern generation is described in U.S. Pat. Nos. 3,537,854; 3,622,742; 3,797,935; 3,925,785; 4,110,594, and 4,422,083.
The present invention uses acousto-optic modulators (AOM) to modulate a laser beam. In these modulators a sound wave propagating in crystal causes diffraction of light, thereby permitting the light to be modulated. This phenomenon has been known for many years and, for example, is discussed in "Acousto-optic Bragg Diffraction Devices and their Applications", by Walter Baronian, IEEE 74 Region 6 Conference, beginning at page 70. The use of acousto-optic modulators for electronic printing is discussed in "Laser Scanning for Electronic Printing", Proceeding of the IEEE, Vol. 70, No. 6, June 1982, beginning at page 597.